piggyback plant vs thousand mothers

piggyback plant

noun
  • A plant native to moist forests near the coast of northwestern North America, Tolmiea menziesii, grown as an ornamental groundcover or container plant for its attractive green foliage with plantlets growing from the bases of the leaves. 

thousand mothers

How often have the words piggyback plant and thousand mothers occurred in a corpus of books? (source: Google Ngram Viewer )